The "Weak Garden of Eden" model for the origin and dispersal of modern humans
(Harpending et
al., 1993) posits that modern humans spread into separate regions from a restricted
source, around
100 ka (thousand years ago), then passed through population bottlenecks. Around
50 ka, dramatic
growth occurred within dispersed populations that were genetically isolated from
each other.
Population growth began earliest in Africa and later in Eurasia and is hypothesized
to have been
caused by the invention and spread of a more efficient Later Stone Age/Upper Paleolithic
technology, which developed in equatorial Africa.

Climatic and geological evidence suggest an alternative hypothesis for Late Pleistocene
population
bottlenecks and releases. The last glacial period was preceded by one thousand
years of the coldest
temperatures of the Later Pleistocene (c. 71-70 ka), apparently caused
by the eruption of Toba,
Sumatra. Toba was the largest known explosive eruption of the Quaternary. Toba's
volcanic winter
could have decimated most modern human populations, especially outside of isolated
tropical
refugia. Release from the bottleneck could have occurred either at the end of
this hypercold phase,
or 10,000 years later, at the transition from cold oxygen isotope stage 4 to warmer
stage 3. The
largest populations surviving through the bottleneck should have been found in
the largest tropical
refugia, and thus in equatorial Africa. High genetic diversity in modern Africans
may thus reflect a
less severe bottleneck rather than earlier population growth.

Volcanic winter may have reduced populations to levels low enough for founder
effects, genetic drift
and local adaptations to produce rapid population differentiation. If Toba caused
the bottlenecks,
then modern human races may have differentiated abruptly, only 70 thousand years
ago.